This invention relates to fluorescent lamps and, more particularly, to annular shaped fluorescent lamps having an improved phosphor coating and to a method for applying the phosphor to improve the performance thereof.
Boric acid, borax or other similar compounds are well known as adhesion promoters for finely divided phosphor coatings applied to the interior surface of fluorescent lamp envelopes. Such adhesion promoting materials are disclosed in U.S. Pat. No. Re. 20,909, dated Nov. 8, 1938 to Claude. In addition, U.S. Pat. Nos. 2,706,691, dated Apr. 19, 1955 to Schaefer; 3,503,780, dated Mar. 31, 1970 to Kamiya; 3,514,276, dated May 26, 1970 to Fugio et al.; and 3,963,639, dated June 15, 1976 to Klein are representative of the highly-developed state of the art with respect to such adhesion promoting coatings. Some of these patents also list as separate adhesion promotors other borates such as sodium borate, see U.S. Pat. No. 3,963,639 at column 1, line 58 thereof.
Fluorescent lamps which have an annular shape are normally first coated with the phosphor material and after the coating operation, the envelopes are bent from a straight configuration to an annular configuration. This has made the coating operation relatively difficult and many of the foregoing adhesion promoting additives have been used in conjunction with such annular shaped lamps, for example, see U.S. Pat. Nos. 3,514,276 and 3,503,780 referred to hereinbefore. Even when adhesion promoters are used with such lamps, it is usually necessary during preparation of the coating suspension to grind the phosphor to a much finer status than is necessary in the case of straight lamps so that it will adhere better and additives such as listed hereinbefore have also been used. The excess grinding results in reducing the brightness of the phosphor and the loss of light output averages about 10% which, while objectionable, is preferable to the phosphor flaking from the coated lamp.